UNSW is a university. Universities offer a range of degrees at different levels, including Bachelor's, Master's, PhD and graduate certificate/diploma. It constitutes one of the more conventional ways of approaching tertiary education (as do other universities).
A college can refer to multiple entities. It could be a part of a university (also known as pathway providers) intended for students who did not achieve a sufficiently high ATAR to study a particular Bachelor's degree (examples include WSU The College, UTS College and TAFE). Provided they intend to eventually study a Bachelor's degree, in order to achieve the requirements to study a Bachelor's degree, such students would need to complete up to a year of education at such institutions. Diplomas (which are different from graduate diplomas in that they are lower qualifications) are offered by such providers.
A college could also refer to a separate institution that offers particular programs in a specialised area. An example of this is the SAE Creative Media Institute, which offers degrees at both undergraduate and postgraduate levels. This is actually an institution to which students can apply through UAC in the same way as other universities/institutions. ICMS is also another example. This institution also offers undergraduate and postgraduate programs targeting business, hospitality management, sport management and international tourism, and is also one that students can apply to through UAC.